Long "Deleting..." Wasn't Stuck!
I ran into this problem, and the, "Deleting..." stage of removing my iPhone's email account took a full hour. I thought it was stuck, but it just took that long.
While it was deleting, I went into my mail app, and I could see outlines of each individual email being deleted one at a time (it was scrolling through), and it did eventually finish, so hang in there and don't despair if you run into this too.
iPhone 6s, iOS 15